We previously reported that adult Ascaris suum possesses
NADH-
metmyoglobin and
NADH-methaemoglobin
reductase systems that are located in the cells of the body wall and in the extracellular perienteric fluid, respectively, which helps them adapt to environmental
hypoxia by recovering the differential functions of
myoglobin and haemoglobin. A. suum
cytochrome b5, an adult-specific secretory
protein and an essential component of the
NADH-metmyo (haemo)
globin reductase system, has been extensively studied, and its unique nature has been determined. However, the relationship between A. suum
cytochrome b5 and the canonical
cytochrome b5 proteins, from the free-living nematode Caenorhabditis elegans is unclear. Here, we have characterised four
cytochrome b5-like
proteins from C. elegans (accession numbers: CAB01732, CCD68984, CAJ58492, and CAA98498) and three from A. suum (accession numbers: ADY48796, ADY46277, and ADY48338) and compared them with A. suum
cytochrome b5 in silico. Bioinformatic and molecular analyses showed that CAA98498 from C. elegans is equivalent of A. suum
cytochrome b5, which was not expressed as a mature
mRNA. Further, the CAA98498 possessed no secretory
signal peptide, which occurs in A. suum
cytochrome b5 precursor. These results suggest that this free-living nematode does not need a haemoprotein such as the A. suum
cytochrome b5 and highlight the crucial function of this A. suum adult-specific secretory
cytochrome b5 in parasitic adaptation.